The mountain apple's entry into Indonesia a century ago still threatens biodiversity there

Around the year 1917, a small, fast-growing tree called Bellucia pentamera was imported from its native habitat in South America to Indonesia's renowned Bogor Botanical Garden. Back then, this island nation was still a Dutch colony, and the Dutch reaped scientific and economic advances from their skilled cultivation of exotic plants, including Bellucia. This tree, known in South America as the mountain apple (manzana de montana), produces edible fruits that were used by at least one Amazonian indigenous group to combat parasite infections.

But the early 20th-century Dutch hunger for mountain apple, and their laissez-faire attitude toward moving plant species around the globe, has had more serious ramifications than anyone predicted in 1917. Not only has Bellucia now persisted for over a century in its adopted Asian environment, it's flourishing. Thanks to its small seeds, which can be transported among Indonesia's 17,000 islands by birds and bats, Bellucia has made the nearly 500-mile journey from its original Bogor planting site to Gunung Palung National Park on the Indonesian side of Borneo.

A perfect storm of factors has enabled this tree to invade Gunung Palung, home to some of the last large tracts of lowland rainforest in Southeast Asia. The national park is a little-known jewel tucked away in the western part of Borneo and harbors seven types of rainforest. Dozens of charismatic and imperiled wildlife species dwell in these diverse jungles. Critically endangered Bornean orangutans roam the park searching for fruit. Sunda pangolins ply the forest floor below them while helmeted hornbills soar through the canopy. Bornean white-handed gibbons and proboscis monkeys leap from enormous strangler figs to old-growth Dipterocarp trees.

But human hands have also left their mark in Gunung Palung. Logging pressure in the park was particularly intense between 2000 and 2002. The selective removal of the biggest, most valuable trees left large gaps in the forest canopy, creating patches on the forest floor with much higher light availability and hotter temperatures than their shaded surroundings. In a previous study published in 2017, a group of scientists found that Bellucia dominated these logged areas of Gunung Palung. Not only could the species withstand the hot sun, but it appeared to out-compete even the native light-loving rainforest species. What, they wondered, made Bellucia so successful in logged areas?

Now, the same scientists, led by PhD student Christopher Dillis of the University of California – Davis, appear to have the answer. For their new paper, published in April in the journal Biotropica, they collected data on the presence or absence of flowers and fruit, as well as the number of ripe fruits, on each Bellucia tree in their Gunung Palung study site over a 13-month period. Another of the study's authors, Andrew Marshall of the University of Michigan, had collected similar data for 200 genera of native rainforest trees between October 2007 and April 2013, so the researchers were able to compare the fruiting frequency (meaning, the proportion of trees producing fruit in any given month) of Bellucia to the native trees.

They may not appear like particularly contentious places, but rainforests are vegetative battlefields. Trees fiercely compete with each other for space, light, water, and nutrients. One way that ecologists think about such competition is termed the "lottery model."

Fruits and seeds are the currency of success in a lottery model. If I were trying to win the lottery, one strategy might be for me to buy as many tickets as I can to maximize my changes of winning. Similarly, the more seeds and fruits that a species (like Bellucia) produces relative to its competitors, the greater its probability of persisting in the forest.

Dillis and his team found that, on average, 56 percent of the Bellucia trees in Gunung Palung National Park produced fruit each month. Just 4 percent of the other rainforest trees did. Even the most frequently-fruiting native genus was far less prolific than Bellucia, with only 25 percent of its trees fruiting each month.

Those were striking results, but another question from Dillis' 2017 research remained unanswered: why did logging open the door for Bellucia to invade Gunung Palung? Canopy gaps are a ubiquitous and well-studied feature of rainforests. They commonly occur when tall trees die and fall to the forest floor. Because they are so common, it seemed strange that Bellucia was drawn to logged areas in particular instead of canopy gaps in general.

It turns out that Bellucia trees growing in gaps created by logging produced more fruits than Bellucia growing in natural canopy gaps. The mechanism behind this is not known, but one possibility is that the intense light availability in logging gaps allows Bellucia seedlings and trees to manufacture large leaves, which translates into increased rates of photosynthesis and more energy available to the tree. It can then allocate this surplus of energy to growth and reproduction, gaining a competitive advantage over native species. Dillis and colleagues observed that in areas where the canopy layer was formed entirely of Bellucia trees, there were virtually no other species present in the understory. They have not yet explored how pervasive this phenomenon is in Gunung Palung. If it does occur frequently and at large scales, it would decrease the plant community diversity in the park, a worrisome scenario given that biodiversity is the key to healthy ecosystems.

What all of this means for the future of Gunung Palung's tropical rainforest remains unclear. I spent two years leading rainforest and orangutan conservation efforts in the Gunung Palung landscape for the Gunung Palung Orangutan Conservation Program, and my experience provides some clues.

Selective logging in Gunung Palung has decreased since the early 2000s, but it continues. Even at the Cabang Panti Research Station, 7.5-mile hike from the nearest village, the harsh buzz of chainsaws occasionally haunts the forest. It's jarring. Juxtaposed against the lush backdrop of tall canopy trees and wildlife, the sound can be downright depressing. However, occasionally the forestry rangers would confiscate chainsaws from people living near Gunung Palung and arrest timber traffickers, and the natural soundscape of the forest would return. And these days, the illegal logging of the biggest and most valuable rainforest trees for the timber industry is by no means the biggest threat to lowland rainforests of western Borneo – it's now been dethroned by large-scale oil palm agriculture, which is itself a complicated conservation issue.

The good news is that unlike other invasive species, it appears that Bellucia cannot run rampant through the jungle without logging gaps. So if logging continues to decrease (or at the very least, does not increase again), then Bellucia populations should follow suit. In the meantime, as Dillis and co-authors note, more research is needed into the effects of Bellucia on the native tree community to understand which plant species are suffering the most.

Current knowledge of Bellucia ecology can also help inform management strategies within Gunung Palung. For example, scientists in Sumatra's Harapan rainforest, which has also been invaded by Bellucia, have found that removing some of the invasive trees has spurred natural forest regrowth. Testing this method at Gunung Palung may be a key to ensuring that the park's lowland tropical forests remain intact and its wildlife persists far into the future.

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The critical topic covered in this piece translates to issues inflicted by the introduction of vigorous invasives globally, although the ramifications of this mountain apple case are particularly severe.

I’m reminded of how privet, a non-native flowering shrub sold in local nurseries, has invaded temperate forests in the United States. People purchase and plant privet because it grows quickly (making it a desirable hedge serving almost-instant gratification) and many find the flowers attractive. The rapid growth of this species in forest understories allows it to out-complete native tree saplings. This is troubling, because as trees grow older and eventually fall down, no saplings will be “lying in wait” to spring up and fill the gap. Forest regeneration, structure, and productivity could drastically change in invaded areas as a result.

And wildlife, such as bald eagles and many species of woodpeckers, that rely on large trees may suffer in the longer term.

On a positive note, the eradication of this invasive species has been shown to improve pollinator populations, even five years post-removal, suggesting that both the mountain apple and privet cases may improve with human intervention/changes in policy.

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